In conjunction with an outdoor tank, typically a large volume tank as often installed in oil refinery plants or the like, one problem known from the past is that rainwater enters the boundary region between the bottom side of the tank and the foundation. The problem is addressed by covering the boundary region between the bottom side of the large tank and the foundation with a pressure-sensitive adhesive butyl-based rubber sheet for preventing the entry of rainwater. However, since the PSA butyl-based rubber sheet is poor in weather resistance, heat resistance and freeze resistance, it fails to prevent the entry of rainwater over a long period of time, allowing rust generation at the bottom of the outdoor tank, with potential tank failure.
For example, Patent Document 1 (JP 3580887) describes a cover member comprising a sealant layer having salt barrier and waterproof properties, a protective layer, and a cover layer stacked. On use, it fails to prevent the entry of rainwater over a long period of time, and sometimes rust generates at the bottom of the outdoor tank.
Also Patent Document 2 (JP 4076673) describes a sealing member having a fold, made of vulcanized EPDM base rubber or unvulcanized rubber base butyl rubber. On use, it fails to prevent the entry of rainwater over a long period of time, and sometimes rust generates at the bottom of the outdoor tank.